There can be advantages gained by fuelling an engine with two different gaseous fuels and having the ability to control the mixture ratio of the two gaseous fuels. A gaseous fuel is defined herein as a fuel that is combustible in an internal combustion engine and that is in the gaseous phase at ambient temperature and pressure.
For example, hydrogen can be mixed with other fuels and burned in the combustion chamber of an internal combustion engine to lower the combustion temperature and thereby reduce the production of NOx. With an engine fuelled with a mixture of hydrogen and natural gas it is possible to extend the lean combustion limit, increase mixture burning speed, and reduce the required ignition energy compared to an engine fuelled with natural gas alone. U.S. Pat. No. 5,787,864, entitled, “Hydrogen Enriched Natural Gas as a Motor Fuel With Variable Air Fuel Ratio and Fuel Mixture Ratio Control” teaches such an approach with a fuel mixture comprising between 21% and 50% hydrogen with the remainder being natural gas. The '864 patent also teaches that the hydrogen and natural gas can be stored in separate containers and that the ratio of natural gas to hydrogen can be varied dynamically and controlled as a function of output emissions and engine power.
Compared to other fuels, hydrogen is at present more expensive so it is desirable to mix hydrogen with a less expensive fuel. If hydrogen is to be added to a fuel for a vehicular engine, an onboard source of hydrogen is required. Storage of hydrogen as a compressed gas can be a problem because of the much larger volume that is required to store a suitable amount of fuel, compared to a conventional liquid fuel with the same amount of energy. Even compared to other gaseous fuels, hydrogen has the lowest energy density. For example, at a storage pressure of about 25 MPa (about 3600 psia), and a temperature of 300 degrees Kelvin (about 27 degrees Celsius or about 80 degrees Fahrenheit), hydrogen has a density of about 17.4 kilograms per cubic meter, and the same amount of energy is available in 48.8 kilograms of diesel fuel, which occupies a volume of less than 0.06 cubic meters, or 41.8 kilograms of methane, which occupies about 0.22 cubic meters at the same storage pressure and temperature. Storage density of gaseous hydrogen can be increased by storing it at higher pressures, but this requires that the fuel tanks be built to withstand such higher pressures and this makes the storage tanks bulky, heavy, and expensive. Another consideration is that some jurisdictions impose regulations that limit the storage pressure for compressed gaseous fuels. Compared to conventional liquid fuels, the storage volume required to store hydrogen in the gaseous phase is higher, even at pressures as high as 70 MPa (about 10,150 psia), and so, for a vehicular application, it can be difficult to find space to store an adequate amount of fuel to give the vehicle a practical range between refueling.
To increase the energy density of hydrogen it is possible to store it in liquefied form. However, liquefying hydrogen is energy intensive and storage of hydrogen as a liquefied gas can also be problematic because of the very low temperatures needed to keep hydrogen in liquefied form, which, depending upon the storage pressure can be at least as low as 20 degrees Kelvin (about −253 degrees Celsius or about −424 degrees Fahrenheit). Because of the very low temperature for storing liquefied hydrogen, there are higher temperature gradients between the storage space and the ambient environment and even a small amount of heat leak into a cryogenic storage container can result in vaporization of some of the liquefied gas. When liquefied gas in a storage vessel is vaporized, if fuel is not consumed quickly enough to reduce the vapor pressure, to maintain vapor pressure below the designed pressure limits of the storage vessel it may be necessary to vent vapor from the storage vessel, which results in fuel being wasted and hydrogen being released into the environment. While technology exists to make a thermally insulated vessel to store liquefied hydrogen for workable hold times, the cost of such a vessel may not be economical for large-scale vehicular and industrial applications.
U.S. Pat. No. 6,397,790 entitled, “Octane Enhanced Natural Gas For Internal Combustion Engine” teaches using a reformer to selectively reform substantially all hydrocarbons in the natural gas except methane to provide a higher octane gaseous fuel comprised of methane, hydrogen and carbon monoxide. With this approach, the onboard source of hydrogen is the natural gas, but the addition of a reforming reactor adds complexity and cost to the fuel system. Exhaust gas from the engine's combustion chambers is directed to a reforming reactor to provide steam and heat for promoting the production of hydrogen by reforming natural gas introduced from the fuel supply into the reforming reactor. The '790 patent also discusses a number of different methods that have been proposed by others for producing hydrogen onboard a vehicle, but as noted in the '790 patent, these approaches all have disadvantages of their own.
Some research has been directed at storing hydrogen as a hydride but practical solutions using this technology have not yet been commercialized. Some of the challenges that currently face the adoption of metal hydride storage systems relate to the weight and the cost of such systems. In addition, loading and unloading can be time consuming, and impurities in the gas could act as a poison that reduces the storage capacity of the system.
It is possible to use an onboard storage vessel that holds a mixture of compressed gaseous hydrogen and natural gas. With this approach only one storage vessel is needed. However, as noted above, the energy density of hydrogen and natural gas stored in gaseous form is very low, even if the gases are stored in a pressure vessel at a high pressure. In addition, when the hydrogen and natural gas are stored as a mixture, it is not possible to control the fuel mixture ratio of hydrogen to natural gas.
Accordingly, while the addition of a second gaseous fuel, like hydrogen, to another gaseous fuel, like natural gas, that is burned in an internal combustion engine can be very helpful in reducing the production of harmful emissions, like NOx, there remain challenges associated with the practical and efficient storage of two gaseous fuels onboard a vehicle.